Second, narrow or broad: Most likely, narrow (much easier to get six votes), with at least four justices saying they would go further, but I’m not counting out the broad decision.

Third, why now? Most important factor: political change in America.

We like to think (and many students are taught) that the federal courts, with life tenure for judges, operate outside the political process, especially when protecting the rights of what the court has called “discrete and insular minorities.” After all, majorities don’t need courts so much. They can win at the ballot box and in the legislature.

But that’s not entirely true. Courts can only stray so far from the constraints (and demands) of politics before their very legitimacy is in question. And legitimacy, at the end of the day, is all the courts have as a basis for the enforcement of their decisions.

The miracle of the rule of law is that we abide by decisions we disagree with, and only rarely (integration in the ‘50s and ‘60s) do we need to resort to the gun-toting National Guard to enforce judicial decisions. But you can only rely on miracles for so much. Think about FDR’s court-packing plan, a reaction to the Supreme Court’s repeatedly striking down New Deal legislation, a plan that Roosevelt dropped when (big surprise) the court changed its mind and “saved nine.”

It was fewer than 20 years ago (1986) when the court upheld Georgia’s criminal sodomy law as applied to consenting gay adults. Seventeen years later (yes, it took 17 years), in an opinion by Justice Kennedy, the court reversed that decision, which puts Justice Kennedy in the odd position (if he votes against gay marriage here) of having decided that gay sex is OK but gay marriage is not, leaving gays as the only group in America who are encouraged — indeed required — to have sex outside of marriage.

While most states continue to limit marriage to a man and a woman, the overwhelming support for this traditional definition has declined precipitously in recent years. The experience of states like Massachusetts, whose Supreme Judicial Court’s decision to legalize gay marriage (based on the state’s Constitution) initially created a political uproar complete with demands for legislative action and the heads of the judges, is instructive. But a funny thing happened in Massachusetts and in similar states: The sky didn’t fall.

If anything, gay marriage was a boost to the local economy. Today, gay marriage is supported by majorities in Massachusetts and across the nation.

States, the court has said many times, should be “laboratories” of experimentation, and they have been in the area of gay marriage. But the question is always: When do the states’ experiences go beyond experimentation and become the basis for a federal mandate?

That really is the broad v. narrow question here. In California, the court could decide that the repeal of the rights of gays to marry — taking away rights from a class of people — presents a different (and easier) case than whether states should be required to afford those rights in the first place.

The Defense of Marriage Act, at issue in the second case before the court, could be thrown out because it is at odds with a federal tradition of deferring to states on the rules of marriage — rules about the age of majority or waiting periods and required tests that have made certain states (think Nevada) marriage magnets.

Chief Justice Roberts’ cousin’s attendance at the oral arguments has received enormous attention because she is a lesbian who lives in California and would like to marry her partner. Everyone knows someone who is gay, she told the press, which was only partly true 20 years ago, when so many of our friends and family members were reluctant to be open about their sexual orientation. I used to joke in the old days, when we were fighting for equal rights for women, that our ace in the hole, more often than we publicly admitted, was how many judges (and justices) have daughters.

Or gay children (Sen. Rob Portman). Or cousins.

This much I know for sure: The wind is blowing in one direction. Gay marriage will be the law of the land, if not this term, then soon. The question for the Roberts court is not whether, but when.